Overskriften på dette indlæg er i sig selv noget vrøvl. Selvfølgelig har coronavirus ikke noget at gøre med troposfærisk udbredelse på VHF og UHF. Men de store nedlukninger af økonomierne har givet nogle afledte effekter på atmosfæren. Flyene er næsten forsvundet og partikel samt NOX forurening er faldet voldsomt.
OZ2M Bo modtog en mail fra DJ6AM hvor han beskrev hvordan tyde amatører havde bemærket markante fald i signalstyrkerne over en længere periode fra OZ7IGY. Dette ledte Bo til at spørge på UK microwave reflektoren om andre havde observeret det samme eller måske havde nogle tanker derom. Svaret fra G4OGI og dialogen med OZ2M kommer omkring grundlæggende udbredelsesforhold som er spændende og interessant læsning. Om der er afledte effekter fra ændring i emissioner og lufttrafik er stadig “up in the air” men der er konstant nye observationer og ideer at lære mere af.
Semper experimentalem

Læs diskussionen herunder.

From:  On Behalf Of Bo, OZ2M via groups.io
Sent: Friday, April 24, 2020 9:04 AM
To: 
Subject: [UKMicrowaves] Bad condx in general?

Hi

I just received a rather interesting email from Joe, DJ6AM, about the current condx and lack of flights:

“We are a bigger group of om here, who test the conditions on the higher bands every day. Some also with PI4. I follow OZ7IGY on 6, 2, and 0,7 m. My 4 m antenna is in Marielyst. At normal times I receive 6 m at 30%, 2 m at 85% and 0.7 m at <10% of the observation time. An om from valley position, about 100 m lower than me, receives only the 2 m beacon (no antennas for the other bands), but at almost 100%.

At the moment we all receive almost nothing. We made the same observation when the Icelandic volcano had erupted and the air traffic had therefore strongly decreased. Do you have any information if this is also true in other directions than south of OZ7IGY?

Do you have reliable information for these observations? I believe that it is the air turbulence or pollution caused by air traffic that amplifies the VHF dx signals.”

Any comments?

Bo

 

From: Nicholas Shaxted
To: Bo, OZ2M ‘Sam Jewell , g4bao
Date: Friday, 24 April 2020, 14:03
Subject: [UKMicrowaves] Bad condx in general? : WARNING: a LOOOng Read

Hi Bo,

A timely note.

I thought I would take some time to reply direct as my post could end up becoming a little long for the attention span of the Microwave Reflector. It also might help to sift some of the counterfactual or anecdotal “evidence” that you no doubt will see. I was going to introduce this topic at Martlesham with a view to expanding on it as data is collected.
Since the start of the COVID-19 emergency I have been lucky enough to maintain a study of propagation conditions across the North Sea. This has allowed me to identify and categorise the different propagation modes across the body of water.My results are based on 10GHz as that is the only band I can run at present. For propagation across the North Sea (or any body of water) the following modes

1) Evaporation duct

2) Surface duct

3) Elevated duct

4) Tropo/Weather scatter

5) Aircraft Scatter

1) Evaporation Ducting

Evaporation ducts seem to be the dominant propagation mode over the southern North Sea. An evaporation duct will form over a body of water as the water vaporises. The rate changes significantly on the differential temperatures, pressure and humidity and beyond the scope of this note. For the southern North Sea the sea temperature reached a minimum of about 6 deg C on 5th March 2020. It has been rising steadily since and is currently sitting about 8 – 9 deg C.
It will rise to around 18 deg in late summer. Monitoring PI7ALK (300km path) has demonstrated strong correlation to sea surface temperature, the higher the temperature the stronger the signal.

Data is suggesting a difference so far of approx. 10dB between the coldest temperature and the 8-9 deg temperature at present. Obviously other factors affect the path (grouped elsewhere as weather effects) therefore like any study collecting and categorising quality of data takes time.

What is clear from the data collective both from Eumetsat/AVIS on SSTs and wave analysis from the ocean institutes (for correlation purposes), evaporation ducting is affected by wave height. At 3cm a wave height over 1.5m with break up the evaporation duct. Actually my calculations indication 1.6m, but as I am dealing with a sea path of 300km a figure of 1.5m is more easily remembered.

An evaporation duct does not have an upper boundary, it gradually reduces depended on local tph (temp, pressure, humidity). Most studies measure up to 20m (the height of surveillance radar on board ships). From an amateur perspective we have a number of coastal beacons sited below a more useful height of 50m. SK6MHI (40m asl), PI7ALK (35m asl) GB3MHL (30m asl), GB3PKT (30m asl). I have been a long proponent of developing two classes of beacon for propagation studies. One lower than 50m for surface effects and ones above for elevated effects. From an amateur perspective evaporation ducts up to about 50m asl over the North Sea appear to affect propagation. It is not unreasonable to assume the same effects are observed with coastal stations bordering the Nordic Seas.

Quite what will happen when the super-sized wind turbines ( 12MW generator capacity on 260m towers with 186m blades). I expect turbulence will affect down to at least 50m. Wind turbines modulate 3cm signals strongly when the blades are side-on. Turbines facing to or from the end of the path seem to have little effect.

We are also see, and this has been the subject of recent activity, strong specular reflections from a single support tower. Work so far suggests that the reflector is either the hub or more probably the nacelle at the top of the tower. There is evidence from signals received from the wind farms Thames Array, Kent Array, London Array and Gunfleet to support this theory. Work is still ongoing. Data from the operational farms off the coast of the Netherlands (Princes Amalia and Egmond aan Zee and Luchterduinen) and PI7ALK strong suggest the orientation of the blades noted above.

When the air temperature is cooler than the sea temperature troposcatter appears to dominate, this is identified as being a weak signal steady signal spread by between 50 and 100Hz with a very narrow acceptance angle in both azimuth and elevation. An error of 0.1 deg will see the signal drop significantly and in many instances it will be lost.

2) Surface Duct

Little work in this area so far as few opportunities, evidence so far suggests this as causes for skewed and blocked paths (Fog and Frontal systems). This not can be expanding but is not too relevant in this note.

3) Elevated ducts

Highly relevant to the question posed. Much time has been spent looking at three recent events.
Analysis of a high pressure system in the last week of December showed few long paths were worked at 3cm. There were considerable path skewing effects which could be attributed to the arrival of a low moving humid air mass as the High pressure system broke down. Air traffic at this time was at normal/high levels.
Two instances of elevated ducts under High pressure systems since the onset of flight restrictions have revealed some interesting observations.

*   Upper level air seems more stable and there is anecdotal evidence that some effects being observed are caused by the huge changes in Dew Point Temperature. A number of weather observations across Europe have recorded Dew Point temperature changes 40 deg or more (+10 to -40) over a very small pressure change which is the equivalent of about 300 – 3500m asl. Exploiting this boundary has seen extremely strong “common volume” scattered signals over paths from 100 – 300km. Data is being collected and I need to step back through historical reading from the same weather stations to ascertain if the boundary layer existed before the flight restrictions.

*   Skewed paths have dominated the past few days under the influence of the Hardanger High. Paths have been skewed more than 4 degrees for prolonged periods with elevation angles exceeding 1 degree. The elevation angle seems to have remained constant across a very wide range of bearings from SK6MHI/OZ1UHF to my North and to F5ZTR/F1BQ in the South. Signal strength being 2 -3 S point lower on the direct horizon headings.
I have not seen such conditions on 3cm for such an extended period of time.

*   Two instances of signal fading took my attention during these events. Both involved the unexplained fading out of a signal from DB0GHZ. The best evidence I have collected so far seems to put the culprit for the path failure on an aircraft crossing an elevated duct. At 2134utc on 18th April 2020 a Boeing 747 crossed perpendicularly the heading between G4OGI (JO01mg) and DB0GHZ (JO34we). The Doppler signature is clear. What follows is a collapse over a period of a few minutes of the signal. Full recovering after about 20 minutes. Images from screen captures are attached and they should all be viewed so as to remain in context with each other. I had a similar event a week or so earlier with the same effect but failed to make sufficient screen captures.

4) Weather/Tropo Scatter

Plenty of evidence is being collected to show the Bow Wave principle ahead of advancing fronts. This is where an air mass is squeezed ahead of an oncoming frontal system creating a short lived refractive bubble.

Often noted in Thunderstorm formation (and recently on the path between G4OGI and F5ZTR) ahead of localised thunder storms. The beacon signal was greatly enhanced and noise level increased by 2dB as the storm cell developed then dissipated once the moisture and electrical charges were released. (I have logged lightning static discharges up to 3.4GHz, 10Ghz is masked by noise level rises from the thermal activity in the cell).

If an aircraft is “large enough” it could in stable air set up its own bow wave of course. The analogy is with a sea vessel make way through a relatively calm sea, the bow wave is often time exploited by Dolphins much to the amusement of humans. I have not noted dolphins in the air in front of aircraft but it does not seem unreasonable to expect something could occur. There is probably an effect more associated with laminar flow as modern aircraft are all equipped  vortex shedding winglets so that it is quite possible that as the air settled a refractive condition occurs in the mixing boundary

Troposcatter is ever present and can be exploited by stations with horizon below about 0.4 deg. The forward scatter phenomenon is best illustrated over sea paths readily observable from 23cm upwards (probably also 70cm). Work is ongoing here with the PI7ALK/PE9GHZ and ON0VHF beacons to better characterise the effects. Need data from other bands to help correlate the 3cm data collected so far.

5) Aircraft scatter

Two clear observations.

Aircraft scatter and subsequent turbulence effects are much reduced. Turbulence may in fact add to scatter angles. The fact that so few aircraft are flying means that the upper levels of the troposphere are much moire stable and long term ducts/ threads can be formed allowing for slowly variable signal enhancements. As noted above turbulence can affect a path for at least 20 minutes. Once you have several aircraft following afterwards the path has no chance to recover. Whilst the forward scatter signal will be disrupted it is highly likely that due to turbulent airflows scatter may happen at larger angles, but even these turbulent airflows will be disrupted by further aircraft movements. This seems to be confirmed by the observations reported.
Several times in the early mornings where upper air is stable (relatively still) and very little aircraft traffic. I, as DL4OGI (JO31jf),  noted on 23cm, 9cnm and 3cm several instances of long term forward scatter signals from GB3MHZ have appeared. Little or no Doppler (drift rather than body velocities) and a characteristic amplitude variation (interference between the moving refractive boundaries)

As the aircraft (monitored on DL4OGI’s own ADS-B receiver) was away from crossing the sight line the conclusion at the time being that this was refraction from Jet efflux. (similar to a condensation tailing suddenly appearing).  Investigations into modern aircraft fuels suggest this is a possibility. Neil G4DBN has probably also observed the same phenomenon. It would be highly unusual to see this effect in a busy aircraft corridor as turbulent wakes would break it up.

I think I had better leave it there. Many points covered.
Much to research, but thanks for pointing out that other people have noted changes to propagation.

Please feel to contact me to discuss or correct any points

As always sorry for the typos…

Mit freundlichen Grüßen; Met vriendelijke groet;
Com os melhores cumprimentos; 敬具;
Le gach dea-ghuí; Venlig Hilsen;
Distinti Saluti; مع جزيل الشكر و التقدير

73 de
Nick – g4ogi from JO01mg

Every breath you take is not only a blessing but a respnsibility

You see things; and you say ‘Why?’ But I dream things that never were; and I say ‘why not?’ ~ George Bernard Shaw (1921)

 

From: Bo, OZ2M Sent: Friday, April 24, 2020 3:57 PM
To: Nicholas Shaxted ; Sam Jewell; g4bao
Subject: Re: [UKMicrowaves] Bad condx in general? : WARNING: a LOOOng Read

Hello Nick

Some very thorough observations and work I must say. Serious work.

It is interesting that the findings that Joe, DJ6AM, and his neighboring OMs have are “signal reduction” ones and not “signal enhancement” types.

In the big picture I don’t know if the air volume is kicked less around than it normally is during the current situation. But what has changed is the CO2 level and perhaps other types of pollution in the air (note from OZ1FDH -changes in CO2 level are less than 1 ppm). OK Europe is far from places like China and India when it comes to air pollution. Even if there is no lid on a duct, but as you describe the media is thinned out vs height the pollution must act in the same way. So could it be that the pollution actually enhances the signals? I guess there is less un-thinned pollution over the North Sea so if there is an impact it might not be experienced there. But from you to Benelux and Germany it might show.

Turbine blades have a lightning wire/mesh that impacts signals when being close by I recall from a customer case while working for Nokia.
https://www.researchgate.net/figure/Lightning-protection-for-large-modern-wind-turbine-blades-31_fig4_280892782

Bo

From: Nicholas Shaxted
To: Bo, OZ2M, ‘Sam Jewell; g4bao
Date: Friday, 24 April 2020, 18:19
Subject: [UKMicrowaves] Bad condx in general? : WARNING: a LOOOng Read

Forget, Yes Bo I should look to restructure the whole piece to emphasise the lack of signals.
Clearly I did not keep that focus as I wrote it. I just hope it provides some food for thought whilst waiting for signal to appear

Regards
Nick